With the development in portable devices such as personal computers and mobile phones, demand is growing for batteries as power sources for such devices.
In batteries for such application, an electrolyte composed of a liquid such as organic solvent has been used as a medium for transferring ions. For this reason, there is a possibility that problems such as leakage of electrolyte from the battery might occur.
In order to solve the above problems, development is under way to produce an all solid state battery using, instead of a liquid electrolyte, a solid electrolyte. An all solid state lithium secondary battery, in particular, is vigorously being studied in many fields as a battery capable of providing a high energy density. This is because Li has a low atomic weight, the greatest ionization tendency, and the lowest reduction potential, and thus, for example, the use of Li metal in a negative electrode active material yields a high electromotive force.
Well-known examples of the solid electrolyte used for the all solid state lithium secondary battery are lithium halide, lithium nitride, lithium oxyacid salts and derivatives thereof. For example, U.S. Pat. No. 5,597,660 reports in the specification that lithium phosphorus oxynitride (LixPOyNz, where x, y and z satisfy x=2.8 and 3z+2y=7.8) obtained by introducing nitrogen (N) into lithium orthophosphate (Li3PO4) has a very high lithium ion conductivity of 1×10−6 to 2×10−6 S/cm although it is an oxide-based material.
When the lithium phosphorus oxynitride is exposed to a wet atmosphere, however, phosphorus atoms (P) forming the lithium phosphorus oxynitride react with water molecules present in the wet atmosphere, during which the phosphorus atoms are reduced to a lower oxidation state from an oxidation state of +5. Thereby, lithium phosphorus oxynitride is decomposed, which significantly decreases the ion conductivity thereof.
When such decrease in ion conductivity occurs in an all solid state battery using a solid electrolyte composed of lithium phosphorus oxynitride, internal impedance will increase. For this reason, its charge/discharge rate characteristics will be significantly impaired.
In view of the above, an object of the present invention is to provide a solid electrolyte capable of preventing the ion conductivity from decreasing even in a wet atmosphere, and an all solid state battery using the solid electrolyte.